Method and apparatus for forming solder balls

ABSTRACT

Methods and apparatus for forming a plurality of uniformly sized solder balls utilize a stencil having a plurality of holes of uniform volume disposed on a substrate. Solder is disposed in the holes of the stencil on the substrate. Typically, the solder is in the form of solder paste which is distributed into the holes using a squeegee. While within the holes of the stencil on the substrate, the solder is melted to form solder balls. The stencil may then be removed to leave the solder balls on the substrate, or the solder balls may be removed while the stencil remains on the substrate.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to electrical circuitryand, more particularly, to methods and apparatus for forming solderballs.

[0003] 2. Description of the Related Art

[0004] In the fabrication of electrical circuits, it is often desirableto couple a circuit component to a substrate. For example, an integratedcircuit chip may be coupled to a ceramic substrate or to a printedcircuit board. A method commonly used for the surface mounting of acircuit component involves the placement of solder balls between thecircuit component and the substrate. The assembly is then heated to meltthe solder balls to couple the circuit component to the substrate.

[0005] To facilitate this type of surface mounting technique, the heightof the solder balls should be substantially uniform. If the height ofthe solder balls is not uniform, the smaller balls may fail to wet,i.e., adhere, to either the circuit component or the substrate, thuscausing the failed formation of the desired electrical connection. Dueto the criticality of the size of the solder balls, various techniqueshave been used in an effort to obtain uniform solder balls.

[0006] Most commonly used methods of forming solder balls employ astencil and operate quite similarly to a screen printing process. Thestencil includes a pattern of holes which are formed in it, and each ofthese holes define a volume that is substantially identical to eachother hole. The hole pattern typically corresponds with a pattern ofelectrical contacts to be formed between the circuit component and thesubstrate.

[0007] In this process, solder is applied in the form of solderparticles, such as tin/lead particles, which are held together in a fluxpaste. The stencil is placed over and slightly spaced above thesubstrate, so that the holes in the stencil are positioned over thecontact pads on the substrate. Once in position, the solder paste isplaced on the stencil. As a squeegee moves along the stencil and overthe holes, it deposits the solder paste into the holes and presses thestencil into contact with the substrate below. When the stencil flexesback into position spaced above the substrate, it leaves plugs of solderpaste on the contact pads of the substrate. After the squeegee hascompleted its stroke along the stencil, the stencil is removed. Thesubstrate is then heated so that the solder reflows to form solder ballswhich adhere to the contact pads.

[0008] The general technique described above suffers from variousdrawbacks. First, the holes in a stencil should be of uniform size andvolume, otherwise the solder balls will not be of uniform size. Second,if a flexible squeegee, such as a rubber squeegee, is used, the squeegeetends to “scoop out” a small portion of the solder paste near the top ofeach hole in the stencil because the flexible material tends to flexdownwardly into each hole. The “scoop out” alters the desired volume ofsolder paste, thus affecting the ultimate size of the solder ballscreated. Third, some of the solder paste tends to stick to the walls ofthe holes of the stencil. Thus, when the stencil flexes back intoposition to leave a plug of solder paste on the substrate, the volume ofsolder paste left on the substrate tends to be less than the volume ofthe hole in the stencil. Indeed, the variations between the plugs ofsolder left on the substrate may be quite significant, thus precludingthe formation of a number of balls having uniform size.

[0009] In an effort to address this first problem, various types ofmethods have been used to create uniform holes in stencils. Forinstance, the holes in some stencils are formed by performing a chemicaletch from both sides of the stencil. While the holes formed in thismanner tend to be quite uniform, chemically etched stencilsunfortunately tend to have holes bordered by small cusps. These cuspstend to retain the solder paste, which eventually plugs the holes in thestencil during repeated use. Alternatively, lasers may be used to cutholes having straight walls in a stencil. Such holes tend to be veryuniform in size and they tend to retain less solder paste than holes cutwith the chemical etching process. Nonetheless, laser cut holes willretain some solder paste and eventually become plugged as well.

[0010] In an effort to address the second problem, relatively inflexiblesqueegees, such as metal squeegees, have been utilized. Althoughrelatively inflexible squeegees solve the problem of “scoop out,” suchinflexible squeegees tend to stretch the stencil, thus slightly alteringthe volume and position of each hole and, eventually, rendering thestencil useless.

[0011] In an effort to address the third problem, E-fab stencils havebeen developed. E-fab stencils have trapezoidally or frustroconicallyshaped holes that widen toward the bottom of the stencil. Due to theshape of the holes, E-fab stencils tend to plug less frequently than thechemical etch or laser cut stencils mentioned above. However, due to themanner in which all stencils are used, some solder paste still sticks tothe walls of the openings as the stencil is removed. Thus, regardless ofthe type of stencil used, the solder paste builds up on the stencilwalls so that the plugs of solder paste remaining on the substrate afterstencil removal become smaller or nonexistent with repeated use of thestencil.

[0012] The present invention may address one or more of the problemsdiscussed above.

SUMMARY OF THE INVENTION

[0013] Certain aspects commensurate in scope with the originally claimedinvention are set forth below. It should be understood that theseaspects are presented merely to provide the reader with a brief summaryof certain forms the invention might take and that these aspects are notintended to limit the scope of the invention. Indeed, the invention mayencompass a variety of aspects that may not be set forth below.

[0014] In accordance with one aspect of the present invention, there isprovided an apparatus for forming solder balls. The apparatus includes asubstrate and a stencil disposed on the substrate. The stencil has aplurality of holes extending therethrough.

[0015] In accordance with another aspect of the present invention, thereis provided an apparatus for forming solder balls. The apparatusincludes a non-wettable substrate having an upper surface and anon-wettable stencil having an upper surface and a lower surface. Thelower surface of the stencil is disposed on the upper surface of thesubstrate. The stencil has a plurality of holes having uniform volumeextending from the upper surface of the stencil through the lowersurface of the stencil.

[0016] In accordance with still another aspect of the present invention,there is provided a system for forming solder balls. The system includesa conveyor belt having a first surface and a second surface and having aplurality of holes in the first surface extending partially through theconveyor belt. A drive device is coupled to the conveyor belt to movethe conveyor belt along a given path. A solder dispensing device isarranged relative to the conveyor belt to dispense solder within theholes in the first surface of the conveyor belt. A heating device ispositioned downstream of the solder dispensing device and arrangedrelative to the conveyor belt to melt the solder within the holes in thefirst surface of the conveyor belt to form a plurality of solder balls.

[0017] In accordance with yet another aspect of the present invention,there is provided a plurality of uniformly sized solder balls fabricatedby disposing solder paste into a like plurality of uniformly sized holesin a stencil disposed on a substrate and by melting the solder paste inthe like plurality of uniformly sized holes to form the plurality ofsolder balls.

[0018] In accordance with a further aspect of the present invention,there is provided a method of forming solder balls. The method includesthe acts of: (a) providing a stencil disposed on a substrate, thestencil having a plurality of holes therein; (b) disposing solder withinthe plurality of holes; and (c) heating the solder disposed within theholes of the stencil disposed on the substrate to form a solder ballwithin each hole.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The foregoing and other advantages of the invention may becomeapparent upon reading the following detailed description and uponreference to the drawings representing exemplary embodiments in which:

[0020]FIG. 1 illustrates a stencil disposed on a substrate;

[0021]FIG. 2 illustrates a cross-section of the apparatus of FIG. 1taken along line 2-2;

[0022]FIG. 3 illustrates an alternate cross-section of the apparatus ofFIG. 1 taken along line 2-2;

[0023]FIG. 4 illustrates another alternate cross-section of theapparatus of FIG. 1 taken along line 2-2;

[0024]FIG. 5 illustrates the cross-sectional view of FIG. 2 during thedeposition of solder paste in the holes of the stencil;

[0025]FIG. 6 illustrates the formation of solder balls in the apparatusof FIG. 1;

[0026]FIG. 7 illustrates a cross-sectional view of the apparatus of FIG.1 taken along line 2-2 illustrating an alternate substrateconfiguration; and

[0027]FIG. 8 illustrates a schematic diagram of an apparatus forcontinuously fabricating solder balls.

DESCRIPTION OF SPECIFIC EMBODIMENTS

[0028] Turning now to the drawings, and referring initially to FIG. 1,an apparatus for forming solder balls is illustrated and generallydesignated by the reference numeral 10. The apparatus includes a stencil12 and a substrate 14. The stencil 12 includes a plurality of holes 16formed therethrough. Unlike the conventional techniques for formingsolder balls discussed above, the stencil 12 is disposed on thesubstrate 14 and it is not removed from the substrate 14 prior to solderball formation. Rather, as discussed in detail below, solder paste isdisposed in the holes 16 of the stencil 12, and the apparatus 10 isheated to melt and reflow the solder paste into solder balls.

[0029] Because the stencil 12 remains on the substrate 14 during theformation of the solder balls, it does not remove paste from thesubstrate 14 that would otherwise be used to form solder balls. Becauseof this, any suitable stencil having uniform holes may be used. Forexample, the stencil 12 may be a laser cut stencil 12A as illustrated inFIG. 2, an E-fab stencil 12B as illustrated in FIG. 3, or a chemicallyetched stencil 12C as illustrated in FIG. 4. It should be noted that agasket (not shown) or similar intermediate layer may be disposed betweenthe stencil 12 and the substrate 14.

[0030] Using the laser cut stencil 12A of FIG. 2 as an example, FIG. 5illustrates the deposition of solder paste 18 into the holes 16A of thestencil 12A. Although any suitable method may be used, the solder paste18 is advantageously placed on the stencil 12A, and a squeegee 20 movesgenerally in the direction of the arrow 22 to deposit the solder paste18 within the holes 16A of the stencil 12A. The solder paste 18 may beof any suitable configuration, such as tin/lead solder particles heldtogether within a flux paste. Regardless of whether the squeegee 20 ismade of a relatively flexible material or a relatively inflexiblematerial, “scoop out” is substantially reduced or eliminated due to thefact that the stencil 12A is not spaced above the substrate 14 andflexed into contact with the substrate 14 during the deposition of thesolder paste 18 on the substrate 14. Furthermore, the use of arelatively inflexible squeegee 20 does not tend to stretch the stencil12A for these same reasons. Thus, either type of squeegee may be usedwithout the disadvantages of either found in the conventionaltechniques.

[0031] As illustrated in FIG. 5, the solder paste 18 deposited in eachhole 16A forms a solder plug 24. Because the holes 16A in the stencil12A have been formed, using known techniques, so that they are quiteuniform with respect to one another, each solder plug 24 is likewiseuniform with one another. The apparatus 10 is then heated to atemperature sufficient to melt or reflow the solder plugs 24 so thatthey form solder balls 26, as illustrated in FIG. 6. It should be notedthat the stencil 12A is advantageously made of a non-wettable material,such as stainless steel or titanium, so that none of the solder adheresto the walls of the holes 16A during the formation of the solder balls26.

[0032] Although the solder balls 26 are typically small, e.g., 10 to 30thousandths of an inch in diameter, and thus quite light, the weight ofthe solder balls 26 may nonetheless cause the bottom of the solder balls26 to flatten against the flat substrate 14. To facilitate thefabrication of more spherical solder balls, a substrate 14A, asillustrated in FIG. 7, having spherical indentations 30 aligned with theholes 16 may be implemented. The spherical indentations 30 conform tothe spherical shape of the solder balls 26 to spread the weight of thesolder balls 26 more evenly to reduce the tendency of the solder balls26 to flatten as they might on a flat substrate.

[0033] Referring again to FIGS. 5 and 6, the holes 16 of the stencil 12may be aligned with respective contact pads 25 on the substrate 14, andthe stencil 12 may be clamped to the substrate 14 during formation ofthe solder balls 26. Once the solder balls 26 have been formed andcoupled to the wettable contact pads 25, the stencil 12 may be unclampedand removed from the substrate 14. It should also be noted that thewidth W_(H) of the openings 16A is larger than the width W_(B) of thesolder balls 26, thus allowing the stencil 12A to be removed from thesubstrate 14 without disturbing the solder balls 26.

[0034] However, if the object of the solder ball formation is to producea mass quantity of uniformly sized solder balls 26, the stencil 12 mayremain clamped to the substrate 14, or even formed integrally with thesubstrate 14. In this instance, the substrate 14 is made of anon-wettable material and does not include wettable pads 25 since thesolder balls 26 are to removed from the apparatus 10 after formationrather than coupled to the substrate 14.

[0035] Although the fixtures described above are quite useful in thefabrication of uniformly sized solder balls, mass production ofuniformly sized solder balls may be facilitated by a more automatedsystem. One example of an automated conveyor-type system is illustratedin FIG. 8 and generally designated by the reference numeral 50. Thesystem 50 utilizes a conveyor belt 52 that generally moves in thedirection of the arrows 54. The conveyor belt 52 includes an uppersection 56 that resembles the stencil 12 discussed above. In otherwords, the upper portion 56 is made of a non-wettable material and itincludes a plurality of uniformly sized holes. The holes areadvantageously spaced quite closely together to maximize the number ofsolder balls formed by the system 50. The conveyor belt 52 also includesa lower portion 58 that is substantially solid and also made of anon-wettable material. The conveyor belt 52 is schematically illustratedas being disposed about and driven by rollers 60 and 62, although itwill be understood that any suitable configuration and drive means maybe used.

[0036] A paste dispenser/receptacle and squeegee 64 deposits solderpaste in the holes of the upper portion 56 of the conveyor belt 52 as itpasses by. Then the conveyor belt 52 loaded with the solder paste entersa furnace 66, which melts the solder paste to form solder balls withinthe holes in the upper portion 56 of the conveyor belt 52. From thefurnace 66, the solder balls continue into a cleaner 68 which removesthe flux used in the formation of the solder balls. As the belt 52continues around the roller 62, a vibrator 70 transfers the solder ballsfrom the conveyor belt 52 into a catch basin 72. The catch basin 72 isadvantageously filled with water to cool the solder balls. The conveyorbelt 52 continues in the direction of the arrows 54 until it reaches thepaste dispenser and squeegee 64 so that the process may be repeated.

[0037] While the invention may be susceptible to various modificationsand alternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the invention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims.

What is claimed is:
 1. An apparatus for forming solder balls comprising:a substrate; and a stencil disposed on the substrate, the stencil havinga plurality of holes extending therethrough.
 2. The apparatus, as setforth in claim 1, wherein the substrate comprises an upper surface onwhich the stencil is disposed, the upper surface of the substrate havinga plurality of wettable pads, wherein each of the plurality of holes inthe stencil are positioned over a respective one of the plurality ofwettable pads.
 3. The apparatus, as set forth in claim 1, wherein thesubstrate comprises a non-wettable material.
 4. The apparatus, as setforth in claim 1, wherein the substrate comprises a printed circuitboard.
 5. The apparatus, as set forth in claim 1, wherein the substratecomprises a ceramic substrate.
 6. The apparatus, as set forth in claim1, wherein the stencil comprises a non-wettable material.
 7. Theapparatus, as set forth in claim 1, wherein the plurality of holes inthe stencil are all of uniform size.
 8. The apparatus, as set forth inclaim 1, wherein the plurality of holes in the stencil are all ofuniform volume.
 9. The apparatus, as set forth in claim 1, furthercomprising solder disposed in each of the plurality of holes in thestencil.
 10. An apparatus for forming solder balls comprising: anon-wettable substrate having an upper surface; and a non-wettablestencil having an upper surface and a lower surface, the lower surfaceof the stencil being disposed on the upper surface of the substrate, thestencil having a plurality of holes having uniform volume extending fromthe upper surface of the stencil through the lower surface of thestencil.
 11. The apparatus, as set forth in claim 10, wherein the uppersurface of the substrate comprises a plurality of wettable pads, whereineach of the plurality of holes in the stencil are positioned over arespective one of the plurality of wettable pads.
 12. The apparatus, asset forth in claim 10, wherein the substrate comprises a printed circuitboard.
 13. The apparatus, as set forth in claim 10, wherein thesubstrate comprises a ceramic substrate.
 14. The apparatus, as set forthin claim 10, wherein the plurality of holes in the stencil are all ofuniform size.
 15. The apparatus, as set forth in claim 10, furthercomprising solder disposed in each of the plurality of holes in thestencil.
 16. The apparatus, as set forth in claim 10, wherein each ofthe plurality of holes in the stencil have a width greater than adiameter of a solder ball formed within each respective hole.
 17. Theapparatus, as set forth in claim 10, wherein the stencil is removablefrom the substrate.
 18. The apparatus, as set forth in claim 10, whereinthe stencil is not removable from the substrate.
 19. A system forforming solder balls comprising: a conveyor belt having a first surfaceand a second surface and having a plurality of holes in the firstsurface extending partially through the conveyor belt; a drive devicecoupled to the conveyor belt to move the conveyor belt along a givenpath; a solder dispensing device arranged relative to the conveyor beltto dispense solder within the holes in the first surface of the conveyorbelt; and a heating device positioned downstream of the solderdispensing device and arranged relative to the conveyor belt to melt thesolder within the holes in the first surface of the conveyor belt toform a plurality of solder balls.
 20. The system, as set forth in claim19, wherein the conveyor belt comprises a first portion and a secondportion, the first portion comprising a non-wettable substrate having anupper surface and the second portion comprising a non-wettable stencilhaving an upper surface and a lower surface, the lower surface of thestencil being disposed on the upper surface of the substrate; thestencil having a plurality of holes having uniform volume extending fromthe upper surface of the stencil through the lower surface of thestencil.
 21. The system, as set forth in claim 19, wherein the conveyorbelt comprises a non-wettable material.
 22. The system, as set forth inclaim 19, wherein the holes in the conveyor belt are all of uniformsize.
 23. The system, as set forth in claim 19, wherein the holes in theconveyor belt are all of uniform volume.
 24. The system, as set forth inclaim 19, wherein each of the plurality of holes in the conveyor belthave a width greater than a diameter of a solder ball formed within eachrespective hole.
 25. The system, as set forth in claim 19, wherein theconveyor belt comprises one of stainless steel and titanium.
 26. Thesystem, as set forth in claim 19, wherein the drive device comprises apair of rollers about which the conveyor belt is disposed, at least oneof the pair of rollers being driven to move the conveyor belt along thegiven path.
 27. The system, as set forth in claim 19, wherein the solderdispensing device comprises a receptacle for holding a supply of solderpaste.
 28. The system, as set forth in claim 19, wherein the solderdispensing device comprises a squeegee positioned adjacent the firstsurface of the conveyor belt to deposit the solder paste within theholes in the first surface of the conveyor belt.
 29. The system, as setforth in claim 19, wherein the heating device comprises a furnacethrough which the conveyor belt passes.
 30. The system, as set forth inclaim 19, further comprising a cleaning device positioned downstream ofthe heating device and arranged relative to the conveyor belt to cleanthe solder balls.
 31. The system, as set forth in claim 19, furthercomprising a catch basin positioned downstream of the heating device andarranged relative to the conveyor belt to receive solder ballstransferred from the conveyor belt.
 32. The system, as set forth inclaim 19, further comprising a transfer device positioned downstream ofthe heating device and arranged relative to the conveyor belt todischarge the solder balls from the conveyor belt.
 33. The system, asset forth in claim 32, wherein the transfer device comprises a vibratorwhich vibrates the conveyor belt to discharge the solder balls from theconveyor belt.
 34. A plurality of uniformly sized solder ballsfabricated by disposing solder paste into a like plurality of uniformlysized holes in a stencil disposed on a substrate and by melting thesolder paste in the like plurality of uniformly sized holes to form theplurality of solder balls.
 35. A method of forming solder balls, themethod comprising the acts of: (a) providing a stencil disposed on asubstrate, the stencil having a plurality of holes therein; (b)disposing solder within the plurality of holes; and (c) heating thesolder disposed within the holes of the stencil disposed on thesubstrate to form a solder ball within each hole.
 36. The method, as setforth in claim 35, wherein act (a) comprises the act of: coupling thestencil to the substrate.
 37. The method, as set forth in claim 36,wherein the act of coupling comprises the act of: clamping the stencilto the substrate.
 38. The method, as set forth in claim 35, wherein act(a) comprises the act of: providing the stencil integral with thesubstrate.
 39. The method, as set forth in claim 35, wherein thesubstrate comprises an upper surface on which the stencil is disposed,the upper surface of the substrate having a plurality of wettable pads,and wherein act (a) comprises the act of: positioning each of theplurality of holes in the stencil over a respective one of the pluralityof wettable pads.
 40. The method, as set forth in claim 35, wherein act(a) comprises the act of: providing the substrate which comprises anon-wettable material.
 41. The method, as set forth in claim 35, whereinact (a) comprises the act of: providing the substrate which comprises aprinted circuit board.
 42. The method, as set forth in claim 35, whereinact (a) comprises the act of: providing the substrate which comprises aceramic substrate.
 43. The method, as set forth in claim 35, wherein act(a) comprises the act of: providing the stencil which comprises anon-wettable material.
 44. The method, as set forth in claim 35, whereinact (a) comprises the act of: providing the plurality of holes in thestencil which are all of uniform size.
 45. The method, as set forth inclaim 35, wherein act (a) comprises the act of: providing the pluralityof holes in the stencil which are all of uniform volume.
 46. The method,as set forth in claim 35, wherein act (a) comprises the act of:providing each of the plurality of holes in the stencil having a widthgreater than a diameter of a solder ball formed within each respectivehole.
 47. The method, as set forth in claim 35, wherein act (a)comprises the act of: providing the stencil and substrate as a conveyorbelt.
 48. The method, as set forth in claim 35, wherein act (b)comprises the acts of: disposing solder paste on the stencil; and movinga squeegee relative to the stencil to dispose the solder paste into theplurality of holes in the stencil.
 49. The method, as set forth in claim47, wherein act (b) comprises the act of: disposing solder paste on theconveyor belt; and moving the conveyor belt along a given path relativeto an adjacent squeegee to dispose the solder paste into the pluralityof holes in the stencil.
 50. The method, as set forth in claim 47,wherein act (c) comprises the act of: moving the conveyor belt along thegiven path through a furnace.
 51. The method, as set forth in claim 39,further comprising the act of: removing the stencil from the substratesubsequent to formation of the solder balls.
 52. The method, as setforth in claim 35, further comprising the act of: removing the solderballs from the stencil and substrate.
 53. The method, as set forth inclaim 35, wherein the acts are performed in order of recitation.